1. Field of the Invention
The present invention relates to a method of manufacturing inkjet recording sheets and a drying apparatus for an application film, and more particularly, to a drying apparatus suitable for drying an ink absorption layer when inkjet recording sheets are manufactured.
2. Description of the Related Art
A step of drying an application film formed after an application liquid is applied to a continuously running web is widely used in the field of manufacturing various application products such as a photographic film, magnetic recording medium and inkjet recording sheets, etc., and is an important step which affects the quality of a product. Especially in the step of drying an ink absorption layer (also called “porous layer” or “color material receptor layer”) when inkjet recording sheets are manufactured, it is well known that the ink absorption layer is generally liable to cracking in a falling rate drying step.
For example, Japanese Patent Application Publication No. 11-348412 describes a problem that when applying a cross-linker, an excessively low water content of an ink absorption layer results in cracking, product loss, causing cracked application film pieces to drop in the step which causes contamination of the step, while an excessively high water content of an ink absorption layer results in the surface of the application film becoming as rough as orange peel. This document discloses a countermeasure which performs drying before the color material receptor layer starts to indicate a falling rate drying speed in such a way that the concentration of a solid content in the ink absorption layer falls within a range of 15 to 40 weight percent.
Furthermore, Japanese Patent Application Publication No. 11-91238 describes that when a large amount of liquid is applied, if drying is performed under a condition of air blowing, part with a non-uniform surface of the application surface contracts differently during drying, which is likely to produce cracking during drying. The document discloses a countermeasure which performs drying with substantially no air blowing until the concentration of the application liquid of the ink absorption layer exceeds 25 weight percent.
However, the problem is that even the drying methods described in Japanese Patent Application Publication No. 11-348412 and Japanese Patent Application Publication No. 11-91238 cannot completely prevent cracking in the ink absorption layer. Drying defects such as cracking, especially fissures or surface roughness may occur at both ends in the direction of the web width of the ink absorption layer and how to prevent these defects is an issue to be addressed.
Furthermore, the measures for preventing drying defects in the width direction of the application film is not limited to an ink absorption layer, and they apply to application films in general and there is a demand for a drying apparatus capable of preventing drying defects in the width direction of the application film.